The field of the present invention pertains generally to data communications equipment, and more particularly to a device for transmitting digital data over a telephone connection.
Data communication plays an important role in many aspects of today's society. Banking transactions, facsimiles, computer networks, remote database access, credits-card validation, and a plethora of other applications all rely on the ability to quickly move digital information from one point to another. The speed of this transmission directly impacts the quality of these services and, in many cases, applications are infeasible without a certain critical underlying capacity.
At the lowest levels, most of this digital data traffic is carried over the telephone system. Computers, facsimile machines, and other devices frequently communicate with each other via regular telephone connections or dedicated lines which share many of the same characteristics. In either case the data must first be converted into a form compatible with a telephone system designed primarily for voice transmission. At the receiving end the telephone signal must be converted back into a data stream. Both tasks are usually accomplished by modems.
A modem performs two tasks corresponding to the needs above: modulation, which converts a data stream into an audio signal that can be carried by the telephone system, and demodulation, which takes the audio signal and reconstructs the data stream. A pair of modems, one at each end of a connection, allows bidirectional communication between the two points. The constraints on the audio signal create the limitations on the speed at which data can be transferred using modems. These constraints include a limited bandwidth and degradation of data by noise and crosstalk. The telephone system typically can carry only signals that range in frequency between 300 Hz and 3,400 Hz. Signals outside this range are sharply attenuated. This range was built into the design of the telephone system since it covers a significant portion of by the human voice spectrum. However, the bandwidth of a channel is one factor that determines the maximum attainable data rate. With all other factors constant, the data rate is directly proportional to the bandwidth.
Another factor is the distortion of the audio signal or any other signal that the communications endpoints cannot control. This includes electrical pickup of other signals being carried by the telephone system (crosstalk), electrical noise, and noise introduced by conversion of the signal from one form to another. The last type will be expanded upon in later discussion.
For general utility, modems are designed to be operable over most telephone connections. Thus, they must be designed for worst-case scenarios, which include bandwidth limitations and significant noise that cannot be removed. Even so, substantial progress has been made on modem design in the past several years. Devices capable of operating at speeds up to 28,800 bits per second are now commonly available. See International Telecommunication Union, Telecommunication Standardization Sector (ITU-T), Recommendation V.34, Geneva, Switzerland (1994) which is hereby incorporated herein by reference. However, theoretical arguments based on the channel bandwidth and noise levels show that the maximum possible speed has nearly been obtained and further significant increases are highly unlikely with the given constraints. This is discussed in C. E. Shannon, "A Mathematical Theory of Communication," Bell System Technical Journal, 27:379-423, 623-656 (1948) which is hereby incorporated herein by reference.
Unfortunately, although speeds approaching 30,000 bits per second (or 3,600 bytes per second) make many data communications applications feasible, conventional modem transmission is still not fast enough for all uses. At these speeds, transmission of text is fast, and low-quality audio, such as digitized speech, is acceptable. However, facsimile or still-image transmission is slow, while high-quality audio is limited and fulll-motion video has not been satisfactorily achieved. In short, what is needed is greater data transmission capability. This is a prerequisite for the new applications and is a necessity for maximizing the performance of many existing applications.
Of course the telephone companies, cable-television providers, and others are not ignorant of these increasing data transmission needs. One approach to providing higher speed data connections to businesses and residences is to provide end-to-end digital connectivity, eliminating the need for additional modems. One offering of such a service is the Integrated Services Digital Network (ISDN). See: International Telecommunication Union, Telecommunication Standardization Sector (ITU-T), "Integrated Services Digital Networks (ISDNs)," Recommendation I.120, Geneva, Switzerland (1993), and John Landwehr, "The Golden Splice: Beginning a Global Digital Phone Network," Northwestern University (1992) each of which is incorporated herein by reference. ISDN replaces the existing analog local loop with a 160,000 bit/second digital connection. Since the bulk of long-distance and inter-office traffic is already carried digitally, this digital local loop can be used for end-to-end digital voice, computer data or any other type of information transfer. However, to achieve these data transmission rates on the local loop, special equipment must be installed at both ends of the line. Indeed, the entire telephone network is currently undergoing a transformation from a voice transmission network to a general data transmission service, with voice just being one particular form of data.
Once installed, each basic ISDN link will offer two data channels capable of 64,000 bits/second, a control channel with a capacity of 16,000 bits/second, reduced call connection time, and other benefits. At these rates, facsimile and still image transmission will be nearly instantaneous, high-quality audio will be feasible, and remote computer connections will benefit from a fivefold speed increase. Some progress toward full-motion video may also be achieved.
The down side of ISDN is its availability, or lack thereof. To use ISDN, the users central office must be upgraded to provide this service, the user must replace its on-premises equipment (such as telephones) with their digital equivalents, and each individual line interface at the central office must be modified to carry the digital data stream. This last step, the conversion to a digital link of the millions of analog connections between every telephone and the central office, is formidable. The magnitude of this task dictates that the deployment of ISDN will be slow and coverage will be sporadic for some time to come. Rural and sparsely populated areas may never enjoy these services.
Another existing infrastructure potentially capable of providing high-speed data communications services is the cable television system. Unlike the telephone system, which connects to users via low-bandwidth, twisted-pair wiring, the cable system provides high-bandwidth connectivity to a large fraction of residences. Unused capacities on this wiring could provide data rates of tens, or even hundreds, of millions of bit per second. This would be more than adequate for all of the services envisioned above including full-motion digital video. However, the cable system suffers from a severe problem--its network architecture. The telephone system provides point-to-point connectivity. That is, each user has full use of the entire capacity of that user's connection--it is not shared with others and does not directly suffer due to usage by others. The cable system on the other hand, provides broadcast connections. The entire capacity is shared by all users since the same signals appear at each users connection. Thus, although the total capacity is high, it is divided by the number of users requiring service. This architecture works well when all users require the same data, such as for cable's original design goal, television distribution, but it does not serve well a community of users with different data needs. In a metropolitan area the data capacity available to each user may be significantly less than via an ISDN or modem connection.
To provide high-speed, data connectivity to a large number of users, the cable system could be modified to isolate different segments of the user population effectively sharing the cable bandwidth over smaller populations. However, like ISDN, this will be a slow, costly process that will provide only partial service for many years to come.
The methods used to design modems are based largely on models of the telephone system that have remained unchanged for several decades. That is, a modem is modeled as an analog channel with a finite bandwidth (400-3400 Hz) and an additive noise component on the order of 30 dB below the signal level. However, a large portion of the telephone system now uses digital transfer of a sampled representation of the analog waveforms for inter-office communications. At each central office, the analog signal is converted to a 64,000 bit/second pulse code modulated (PCM) signal. The receiving office then reconstructs the analog signal before placing it on the subscriber's line. Although the noise introduced by this procedure is, to a first approximation, similar to that observed on an analog system, the source of the noise is quite different. See K. Pahlavan and J. L. Holsinger, "A Model for the Effects of PCM Compandors on the Performance of High Speed Modems," Globecom '85, pages 758-762, (1985), which is incorporated herein by reference. Most of the observed noise on a telephone connection that uses digital switching is due to quantization by the analog-to-digital converters needed to convert the analog waveform into a digital representation.
As noted above, most telephone connections are currently carried digitally between central offices at rates of 64,000 bits/second. Furthermore, ISDN services demonstrate that it is possible to transmit significantly more than these rates over the local loop. It has been suggested that it may be possible to design a transmission scheme that takes advantage of these factors. Kalet et al. postulate a system, shown in FIG. 2, in which the transmitting end selects precise analog levels and timing such that the analog-to-digital conversion that occurs in the transmitter's central office might be achieved with no quantization error. I. Kalet, J. E. Mazo, and B. R. Saltzberg, "The Capacity of PCM Voiceband Channels," IEEE International Conference on Communications '93, pages 507-511, Geneva, Switzerland (1993), which is incorporated herein by reference. By making use of the mathematical results of J. E. Mazo it is conjectured that it should be theoretically possible to reconstruct the digital samples using only the analog levels available at the receiver's end of the second local loop in the communications path. J. E. Mazo, "Faster-Than-Nyquist Signaling," Bell System Technical Journal, 54:1451-1462 (1975), incorporated herein by reference. The resulting system might then be able to attain data rates of 56,000 to 64,000 bits/second. The shortcoming of this method is that it is nothing more than a theoretical possibility that may or may not be realizable. Kalet et al. state that "This is a hard practical problem and we can only conjecture if a reasonable solution would be possible." Id. at page 510.
An example of a conventional attempt to solve the foregoing problem is found in work by Ohta, described in U.S. Pat. Nos. 5,265,125 and 5,166,955, which are hereby incorporated by reference. Ohta disclosed an apparatus to reconstruct a PCM signal transmitted through a communications channel or reproduced from a recording medium. These patents exemplify some conventional techniques abundant in the literature to deal with the general problem of reconstructing a multi-valued signal that has passed through a distorting channel. See also, for example, Richard D. Gitlin, Jeremiah F. Hayes and Stephen B. Weinstein, "Data Communications Principles," Plenum (1992), incorporated herein by reference. However, such conventional teachings do not consider the application of methods to handle the output from a nonlinear quantizer, nor do they deal with the specific problems of decoding digital data passed over a telephone local loop. Furthermore, the problem of reconstructing a sampling rate clock from the PCM data is non-trivial when the PCM signal can take on more than two values. For example, in the patents by Ohta, a simple clock recovery scheme which relies on a binary input signal is employed. This type of clock recovery cannot be used with the multivalued codes used in a telephone system. Also, compensation for drift with time and changing line conditions requires use of an adaptive system which the prior art of PCM reconstruction does not include.
Thus, there is currently a critical disparity between the required or desired data communications capacity and that which is available. Existing modems do not provide adequate capacities, and new digital connectivity solutions are several years away from general availability. Refitting the existing infrastructure with ISDN capability is a sizable task and may take a decade before its use is widespread. A new method of data transmission could immensely benefit many current applications as well as making several new services available which would otherwise have to wait until the infrastructure catches up with the requirements.
Accordingly, there is a need for providing a new system of data transfer which provides the capability to receive data at high rates over existing telephone lines.
There is also a need for an improved system of data transfer which can enable systems, equipment, and applications designed for a digital telephone system (such as ISDN) to be used with analog connections.
There is also a need for an improved system of data transfer which is capable of taking advantage of the digital infrastructure of the telephone system without requiring costly replacement of all subscribers' lines.
It also would be desirable to create a high speed communication system to provide a means to distribute high-quality digital audio, music, video, or other material to consumers. Such an improved system of data transfer would advantageously provide a means to distribute, on-demand, individually-tailored information, data, or other digital material to a large number of consumers.
There is also a need for an improved high speed communications system to provide greater throughput for commercial applications such as facsimile, point-of-sale systems, remote inventory management, credit-card validation, wide-area computer networking, or the like.